Finding a Good Coupler

4504 Views 50 Replies 7 Participants Last post by 57Chevy, Mar 23, 2022

sifawangiaEV

Discussion Starter · Feb 8, 2022

Hi everyone,

I'm in the process of choosing a coupler to mate my Warp9 DC motor with my Mini Cooper manual transmission and I've come across a couple of options. I'm browsing grainger.com and their Rigid Couplers here and I've come across a few coupling types that I think will work for my conversion. Here are the different types (on the grainger.com website):

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I've heard that these types of couplings have been successful in EV conversions so I thought I might give one a try. I'm leaning towards using either the 1-piece clamp rigid shaft coupling with a 1.125 inch bore diameter:

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Link (https://www.grainger.com/product/RULAND-MANUFACTURING-Rigid-Shaft-Coupling-1-1-8-2ALJ3?Pid=search)

Or using the 1-piece set screw rigid shaft coupling with a 1.125 inch bore diameter:

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Link (Rigid Shaft Coupling: 1 1/8 in Bore Dia., Steel, 1 7/8 in Outside Dia.)

Are their any benefits/drawbacks to using one coupling over the other? I'm thinking of having a machine shop cut and weld my clutch plate onto the transmission side of the coupling. What does everyone think of this? What coupling do you recommend?

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sifawangiaEV

Discussion Starter · #21 · Feb 23, 2022

sifawangiaEV said:
Sorry everyone if I've been all over the place but I've decided to go with a rigid coupling because it would be the longest lasting for my conversion and I'm willing to do the extra tough work to make it work. I just dropped it off at the machine shop with my clutch plate to be welded together, when I realized something not so good...the torque of the rigid coupling wasn't as high as I thought it was. Here's the spec sheet:
View attachment 127750
I could've sworn that when I bought the item, it said that it had a torque rating of 2,400 in-lb. I'm thinking there might be a glitch on the website but I'm not certain because there is a chance that I somehow read it wrong. What do you all think? Does this rating look right for this coupling or does it seem like a glitch? Should I be considering a new coupling?
View attachment 127750

Update: there was an error on the website. The torque rating is actually 4,400 in-lb.

KiwiME

· #22 · Feb 24, 2022

Just send a dimensioned drawing to Rexnord and have them size and configure it for you.

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sifawangiaEV

Discussion Starter · #23 · Feb 25, 2022

The machine shop that I sent my rigid coupling and clutch plate to has finished welding the two parts together. Here's the picture that they sent me:

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I'm going clutchless which is why I'm just using the hub and not the clutch plate. What does everyone think of the design? If aligned properly, will it hold? Are there any modifications they should make or is it good?

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57Chevy

· #24 · Feb 25, 2022 (Edited)

put it on your input spline and spin the gearbox over, does it wobble? Put it on your motor and spin that over, how much does the welded bit wobble? Put a DTI on it and work out how far out it is. It will be out of alignment to some degree, how much determines how long the spline and/or coupling will last

It looks like they bead blasted it, give it a thorough disassembly and clean with brake clean or similar first and then blow out with compressed air

KiwiME

· #25 · Feb 25, 2022 (Edited)

They did a nicer job than I expected but I'd be impressed if it lasts, even if the parts are concentric.
1) there is evidence of shrinkage along the middle of the weld.
2) the coupling split will cause a stress concentration where it intersects the weld. (just below the arrowhead)

I would have welded up the coupling split on the gearbox half to stop movement at the weld. At least shim the gap tight and install some screws to limit flexing of that half.

In your favour, the gearbox input shaft will have some some wiggle tolerance and it's unlikely anything expensive will be damaged if the coupling fails. Grease the splines lightly at installation and make sure that the key(s) are a tight fit in the keyways.

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brian_

· #26 · Feb 25, 2022

KiwiME said:
Just send a dimensioned drawing to Rexnord and have them size and configure it for you.
View attachment 127792

That looks like a really nice coupling line, although not fundamentally different from other rubber flex joints (or giubos).
The rubber is available in a range of hardness, but any of them will be torsionally much softer than required; maybe that's harmless.
It can handle some misalignment, but every rotation of the shaft is a cycle of distorting the rubber, by an amount dependent on misalignment. I don't know how big one of these things would need to be to withstand highway travel, but I didn't look at the specs in detail.

I'm inclined to like these just on a sentimental basis - my father once worked for Rexnord.

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brian_

· #27 · Feb 25, 2022

KiwiME said:
...
2) the coupling split will cause a stress concentration where it intersects the weld.

I would have welded up the coupling split on the gearbox half to stop movement at the weld. At least shim the gap tight and install some screws to limit flexing of that half...

This is an excellent observation.

The original coupler is designed to clamp to each shaft, but in this modification the transmission side uses a spline instead, so the clamping action won't be used at all... and is completely incompatible with the rigid splined insert welded all the way around.

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KiwiME

· #28 · Feb 25, 2022 (Edited)

brian_ said:
... but every rotation of the shaft is a cycle of distorting the rubber, by an amount dependent on misalignment.

An industrial coupling is a professional solution for the very-common motor to gearbox application. Properly sized these would last a long time. There are other types of less-flexible material of course; I only posted that particular photo to show that a spline can be machined into one half. The other half would use a tapered lock sleeve around the keyed shaft. Decoupling motor interia from the gearbox is never a bad thing as long as resonances are avoided.
I worked as an engineer in the field of engine and powertrain test equipment for a few years in the UK, 1980s, Froude Engineering.

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brian_

· #29 · Feb 25, 2022

KiwiME said:
An industrial coupling is a professional solution for the very-common motor to gearbox application. Properly sized these would last a long time.
...
Decoupling motor interia from the gearbox is never a bad thing as long as resonances are avoided.

Sure, but no one is engineering this system of interacting inertial objects, so torsional compliance in the coupling could create resonant problems, rather than solve them... and no production EV designer finds a need to decouple the motor inertia from the transmission. At least the hysteresis of the rubber will tend to damp out problems, unlike the steel coil springs of a typical clutch hub.

DMPstar

· #30 · Feb 25, 2022 (Edited)

I'm adding my 2c not because I am an expert in this field, but because I made critical errors in this step of my conversion project. My transmission started leaking from the input shaft within a few blocks of the first test drive, and I drove the conversion less than 50 miles before scrapping the project due to lack of time, space, and money to redesign the adapter assembly.

57Chevy said:
put it on your input spline and spin the gearbox over, does it wobble? Put it on your motor and spin that over, how much does the welded bit wobble? Put a DTI on it and work out how far out it is. It will be out of alignment to some degree, how much determines how long the spline and/or coupling will last

Agreed, use the dial indicator when the coupler is attached to the motor. Looking at wobble with the coupler slid onto the trans input shaft may be misleading and difficult depending on the transmission design, as well as the aforementioned clutch disc spline-slop factor. Also as mentioned earlier, transmissions that use a pilot bearing can be a bit wobbly at the input shaft when detached from the accompanying engine.

sifawangiaEV: Can you confirm if your crankshaft has a pilot bearing or not?

It appears your project may be moving faster than the advice can come in and be appreciated. The momentum of an exciting DIY build can be great, but can also corner you if something gets overlooked or compromised. You have been given some legitimate and free Mechanical Engineering advice in the above posts, so make sure you understand the concerns that have been raised. I ignored or glazed over some advice from experienced members here and got to see the results, even though myself and the machinist agreed on the feasibility of the adapter design.

Can you get the machinist to run the piece with a dial indicator, and show you how the parts were made centric for the welding? Did they put machine screws in the threads and torque them onto some properly sized rod for that portion? If you did not provide them with a runout spec or some measurable expectations then you probably cannot expect them to compensate you, but you may be able to save damage to your gearbox by stepping back and taking some time to confirm some things.

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KiwiME

· #31 · Feb 25, 2022 (Edited)

brian_ said:
Sure, but no one is engineering this system of interacting inertial objects,...

Perhaps they should.
Designed EVs have larger gears and splines than were seeing here. This is a fairly flimsy setup in comparison and I'd see softening shock loading as an advantage.

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sifawangiaEV

Discussion Starter · #32 · Feb 25, 2022

KiwiME said:
1) there is evidence of shrinkage along the middle of the weld.
2) the coupling split will cause a stress concentration where it intersects the weld. (just below the arrowhead)

That's a good point. I did notice that split and wondered if that would be an issue. What should I tell the machine shop to do to fix this issue? Should they just reweld it?

KiwiME said:
I would have welded up the coupling split on the gearbox half to stop movement at the weld. At least shim the gap tight and install some screws to limit flexing of that half.

Are you saying that the clutch bit should be inserted deeper into the coupling? Because I believe that it partially is but I'll have to see it to find out. The person at the machine shop that I was talking to mentioned using screws and a key with the transmission side spline so that the coupler can work as intended but he later decided to have some of the clutch hub part in the coupler and weld it on.

brian_ said:
The original coupler is designed to clamp to each shaft, but in this modification the transmission side uses a spline instead, so the clamping action won't be used at all... and is completely incompatible with the rigid splined insert welded all the way around.

Yes, the clamping wouldn't be used on the transmission side but it would be used on the motor side. I'll still be able to use a key and the screws and clamp down on the motor shaft.

DMPstar said:
@sifawangiaEV: Can you confirm if your crankshaft has a pilot bearing or not?

I don't think that it has a pilot bearing. I attached a picture of the clutch disc in my starting post so you can see that too for confirmation.

DMPstar said:
Can you get the machinist run the piece with a dial indicator, and show you how the parts were made centric for the welding? Did they put machine screws in the threads and torque them onto some properly sized rod for that portion? If you did not provide them with a runout spec or some measurable expectations then you probably cannot expect them to compensate you, but you may be able to save damage to your gearbox by stepping back and taking some time to confirm some things.

I believe that they did torque the screws to a rod, but I'll check in on Monday when the shop is open to find out what they did and if they can modify it if it needs fixing. If the part is determined to be concentric, and the split in the welding is fixed, am I in the clear?

Thanks everyone for the input. It is greatly appreciated, considered, and very very helpful.

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57Chevy

· #33 · Feb 25, 2022

If the weld cracked on the first go then a second go over the top isn't the solution. They need to find out why; is it incompatible materials, incompatible filler rod, contamination, technique... That crater in the end pool looks ominous, it likely has porosity right down to the root and could initiate a crack anyway, even if that line isn't a crack.

Welding axles is an age old practice that often results in failure. Some people have the knack of it, many don't. You are doing a very similar thing to welding an axle, just less torque.

sifawangiaEV

Discussion Starter · #34 · Feb 25, 2022

57Chevy said:
If the weld cracked on the first go then a second go over the top isn't the solution. They need to find out why; is it incompatible materials, incompatible filler rod, contamination, technique... That crater in the end pool looks ominous, it likely has porosity right down to the root and could initiate a crack anyway, even if that line isn't a crack.

Welding axles is an age old practice that often results in failure. Some people have the knack of it, many don't. You are doing a very similar thing to welding an axle, just less torque.

Ok, that makes a lot of sense. I'll talk to them about what the cause might be and if they can't fix it, I'll take it to a different machine shop and see if they can figure it out/do it right.

Also I cannot find the specs for this coupling that specify how much to torque the screws...Is there a general rating out there somewhere?

KiwiME

· #35 · Feb 25, 2022 (Edited)

Presumably they've machined the clutch hub end (the end not visible) to fit the coupling so that it's centered but now that it's welded there's no reason to allow that side of the coupling to flex.
I was suggesting welding it along the red line but you'd want to grind a "V" first so that the weld can take good hold of the parent steel.

But before spending the money, perhaps check runout first in case it's not concentric and needs to be redone. If the minor diameter of the spline and/or the outside diameter is more than 0.002" out radially I'd scrap it. Oh, and that's when clamped to the motor shaft as mentioned by others.

I'd sure like to see photos of the actual gearbox input shaft and motor output shaft. And some idea of the spacing between them you're expecting.

I'll just add that my recommendation remains to use an industrial flexible coupling. That still requires the two sides to be well-aligned, perhaps 2-3 thousandths TIR.

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remy_martian

· #36 · Feb 25, 2022

You've shown that the upper half of the black coupler is useless and what he has is going to be an out of balance mess.

That weld is cold, almost looks like it was "aluminum welded" vs TiG welded using one of those special rods that works with MAPP gas. That weld makes that assembly a writeoff. Can't be fixed for service.

Next go...cut the black coupler in half to where only two cinch bolts are used and a sliver above the factory gap.

Groove the spline piece and the new baby two bolt coupler for welding HALF way around, like its amputated friend was joined.

Get it TiG welded, several passes, then chuck it in a lathe and fillet the weld so it's balanced and there are no stress points (sudden geometry changes...ridges, etc). You may need to take it to a machine shop and a competent welder vs guys who do joints on irrigation stuff.

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DMPstar

· #37 · Feb 25, 2022

KiwiME said:
If the minor diameter of the spline and/or the outside diameter is more than 0.002" out radially I'd scrap it.

👍
A few thousandths is the actual tolerance here as I have learned.

KiwiME said:
I'd sure like to see photos of the actual gearbox input shaft and motor output shaft. And some idea of the spacing between them you're expecting.

👍

sifawangiaEV said:
I don't think that it has a pilot bearing. I attached a picture of the clutch disc in my starting post so you can see that too for confirmation.

It is hard to determine if your gearbox used a pilot bearing based on pictures of the clutch disc. Gotta have pics of those shafts as KiwiME said, short of us doing research into your car's design.

sifawangiaEV said:
Also I cannot find the specs for this coupling that specify how much to torque the screws...Is there a general rating out there somewhere?

Specified screws on the product snip you posted were 1/4-28. You should be able to find a torque spec for that. Grade 8 would be a good estimate, you shouldn't break or strip them in that range.

remy_martian said:
You may need to take it to a machine shop and a competent welder vs guys who do joints on irrigation stuff.

ohhh r_m, sometimes getting to the point using restraint.

sifawangiaEV

Discussion Starter · #38 · Feb 25, 2022

KiwiME said:
I'd sure like to see photos of the actual gearbox input shaft and motor output shaft. And some idea of the spacing between them you're expecting.

The other end of the coupler that's hidden fits the motor shaft and part that's welded fits the transmission shaft. Here are the pictures:

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KiwiME said:
I was suggesting welding it along the red line but you'd want to grind a "V" first so that the weld can take good hold of the parent steel.

I'm not a welder so I'm having trouble understanding what you mean. Are you saying that the coupling should fit in deeper? Or are you saying to weld through that gap for a better grip?

remy_martian said:
Next go...cut the black coupler in half to where only two cinch bolts are used and a sliver above the factory gap.

Groove the spline piece and the new baby two bolt coupler for welding HALF way around, like its amputated friend was joined.

Get it TiG welded, several passes, then chuck it in a lathe and fillet the weld so it's balanced and there are no stress points (sudden geometry changes...ridges, etc). You may need to take it to a machine shop and a competent welder vs guys who do joints on irrigation stuff.

Sorry but I don't really understand what you're telling me to do. So I cut the coupler in half and then what do I do? How will this be different from what I have already? Is the only difference a different weld (TiG vs aluminum)?

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57Chevy

· #39 · Feb 25, 2022

Maybe it'd be a better shot welding the clutch spline to a stump of shaft the same diameter as the motor shaft, then tighten the coupler on both the motor and the stump shaft how it was intended to work

You can find generic torque specs for the fasteners online. They look about 1/4 unf

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remy_martian

· #40 · Feb 25, 2022

^ This. My daughter called and interrupted me typing this very same suggestion up.

The problem might be that there won't be enough meat over the splines to get to motor diameter, but that just means reaming half the coupler to a larger ID...but only if they don't already stock couplers in two different ID's.

No welding needed..

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